Porcine reproductive and respiratory syndrome virus (PRRSV) is a member of the virus family Arteriviridae and belongs, together with the Coronaviridae, to the virus order Nidovirales. PRRSV is an enveloped virus with a single-stranded, positive-sense RNA genome of about 15 kilobases comprising nine open reading frames (ORFs), namely ORF1a, ORF1ab, ORF2a, ORF2ab, and ORFs 3 through ORF7. ORFs 1a and 1ab encode large polyproteins that are processed into the viral nonstructural proteins (nsp) by auto- and transcleavages of viral proteases nsp1, nsp2, and nsp4 (Snijder and Meulenberg, 1998). ORF4 encodes a minor glycoprotein (GP4) which is, next to a major glycoprotein (GP5) and two other minor glycoproteins (GP2a and GP3), found in the viral envelope, wherein all of said glycoproteins are important for infectious virus production.
PRRSV is considered one of the economically most important infectious agents in pigs causing late-term reproductive failure in sows and respiratory disease in growing pigs. Often, PRRSV infection is complicated by secondary bacterial infections being attributed to the immunosuppressive nature of the virus. Also, PRRSV viremia lasts for weeks, and virus then still can be detected in lymphoid organs for several months, demonstrating difficulties or failure of the host's immune response to clear the virus (Allende et al., 2000).
There are two distinct viral PRRSV genotypes causing similar clinical symptoms that diverge by about 40% on nucleotide sequence level, genotype I (EU) and genotype II (US). The North American (US) prototype strain is VR-2332, while the European (EU) prototype strain is Lelystad virus.
However, in a first consideration, as PRRS virus strains have a high biological diversity and evolve rapidly on individual farms (Badaoui et al. BMC Veterinary Research 2013, 9:58), new PRRSV isolates are needed for a better understanding of PRRS, for reproducing said disease in its different forms, for comparative tests, and as platform for the development of new vaccines, medications and diagnostics for the prophylaxis, treatment and diagnosis of PRRS.
In a second consideration, a growing number of infectious cDNA clones of the PRRS virus are becoming available to the scientific community, most of which are based on the US type of the virus. For the EU type, however, only few clones are available. Thus, there is a strong need for new infectious cDNA clones of European (genotype I) PRRS virus, for a better understanding of PRRS, for comparative tests, as platform for the development of new vaccines, medications and diagnostics for the prophylaxis, treatment and diagnosis of PRRS, wherein the use of the cDNA clone results in a high yield of virus production. Thus, for experimental convenience in the PRRS vaccine research an infectious cDNA clone would be needed enabling the production of genotype I PRRS virus in high amounts